The hunt for missing Malaysia Airlines Flight MH370 could take “decades”, the company’s British commercial chief has admitted.

Hugh Dunleavy, a former Ministry of Defence employee who became the airline's director of commercial operations in 2012, said he believed something “untoward” had happened to the Boeing 777, which disappeared on March 8 as it flew from Kuala Lumpur to Beijing with 239 people on board.

“Something untoward happened to that plane. I think it made a turn to come back, then a sequence of events overtook it, and it was unable to return to base,” Mr Dunleavy told the Evening Standard.

“I believe it’s somewhere in the south Indian Ocean. But when [a plane] hits the ocean it’s like hitting concrete. The wreckage could be spread over a big area. And there are mountains and canyons in that ocean. I think it could take a really long time to find. We’re talking decades,” he added.

In the days immediately after the plane went missing, a senior Malaysian minister told The Telegraph he feared the search mission would take weeks or months.

“We are in for the long haul,” Hishammuddin Hussein, the acting transport minister, said.

However, even that prognosis proved wildly optimistic. More than three months have now passed since the plane disappeared and no physical trace of it has been found despite a massive multi-national search effort.

On Sunday it emerged that Zaharie Shah, the plane’s captain, had used a flight simulator at his home to plot a flight path to a remote island deep into the southern Indian Ocean, where much of the recent search operation has been focused.

That discovery has rekindled suspicious that the pilot may have hijacked the plane and deliberately steered it off course.

Hishammuddin Hussein, the minister, rejected suggestions that the pilot was now “prime suspect” on Monday. “We should not entertain conspiracy theories,” he told Malaysia’s The Star newspaper.

Mr Dunleavy, who is originally from Ealing, defended his company’s initial response to MH370’s disappearance.

“People say, ‘Why didn’t you work quicker?’ But you’re calling pilots, explaining the situation, waiting for them to send out pings, doing the same to the next plane, then the next, and it’s four in the morning, you don’t have 50 people in the office, only a couple. An hour goes by frighteningly quickly — you realise that the missing plane is now another 600 miles somewhere else.”

However, the airline’s commercial boss criticised the Malaysian government for taking so long to reveal that the missing plane had turned back over the Malay peninsula towards the Strait of Malacca.

“I only heard about this through the news,” Mr Dunleavy said. “I’m thinking, really? You couldn’t have told us that directly? Malaysia’s air traffic control and military radar are in the same freakin’ building. The military saw an aircraft turn and did nothing.”

“They didn’t know it was MH370, their radar just identifies flying objects, yet a plane had gone down and the information about something in the sky turning around didn’t get released by the authorities until after a week. Why? I don’t know. I really wish I did.”